Various types of combustion may be used in an internal combustion engine. For example, a diesel cycle may be used where diesel fuel is injected into the combustion chamber and combustion occurs upon injection. Another type of combustion may be referred to as homogeneous charge compression ignition (HCCI) where fuel is injected into the combustion chamber and mixes with air near the autoignition temperature of the fuel to form a substantially homogeneous air-fuel mixture. Combustion of the air-fuel mixture occurs during compression when the temperature of the combustion chamber exceeds the autoignition temperature of the air and fuel mixture. HCCI operation can be used to provide greater fuel efficiency and reduced NOx production under some conditions. Yet another type of combustion may be referred to as partially homogeneous charge compression ignition (pHCCI), which is similar to HCCI except the injected fuel is only partially mixed with the air, thus creating a partially homogeneous air-fuel mixture.
One approach to controlling combustion is U.S. Pat. No. 6,276,334. In one example, during an HCCI operation, a single injection of fuel is performed late in the cycle in order to delay autoignition so that substantial mixing of the air and fuel occurs. In another example, split injections are performed where the first injection occurs substantially early in the intake stroke so that noise is reduced.
The inventors herein have recognized a disadvantage with such an approach. In some conditions, it may be desirable to delay autoignition as long as possible to provide sufficient time for mixing of air and fuel. However, when using a single injection to fuel the combustion chamber, a substantially large amount of fuel may be burned within a relatively short time period (such as with high engine loads), thus a high level of noise may be generated. Alternatively, if a split injection strategy is utilized where even a small early injection (pilot injection) is performed before a substantially larger main injection of fuel, the temperature within the combustion chamber may rise, thus advancing autoignition. As autoignition is advanced, the amount of time available for the air and fuel to mix is reduced and the amount of NOx and soot emissions may increase.
In one approach, the above issues may be addressed by a method of operating an internal combustion engine having a combustion chamber with a piston, the internal combustion engine capable of injecting fuel into the combustion chamber multiple times during a cycle, the method comprising: performing a first fuel injection after approximately −25 crank angle degrees after top dead center and before approximately 15 crank angle degrees after top dead center; and performing a second fuel injection at least 5 degrees after the start of the first fuel injection and less than approximately 25 crank angle degrees after the start of the first fuel injection.
In this way, combustion can be delayed by performing a first injection after approximately −20 degrees CA ATDC thus providing time for the air and fuel to mix. Further, by using a second injection performed approximately 10 degrees CA after the first injection, it is possible to reduce noise generated by the engine (by reducing rate of pressure rise, even though combustion is delayed), without prematurely raising combustion temperature. Therefore, it is possible to reduce engine noise concurrently while reducing emissions and improving fuel efficiency.